Sixteen months into the uprising in Syria, the United States is struggling to develop a clear understanding of opposition forces inside the country, according to U.S. officials who said that intelligence gaps have impeded efforts to support the ouster of Syrian President Bashar al-Assad.

U.S. spy agencies have expanded their efforts to gather intelligence on rebel forces and Assad’s regime in recent months, but they are still largely confined to monitoring intercepted communications and observing the conflict from a distance, officials said.

Interviews with U.S. and foreign intelligence officials revealed that the CIA has been unable to establish a presence in Syria, in contrast with the agency’s prominent role gathering intelligence from inside Egypt and Libya during revolts in those countries.

With no CIA operatives on the ground in Syria and only a handful stationed at key border posts, the agency has been heavily dependent on its counterparts in Jordan and Turkey and on other regional allies.

The lack of intelligence has complicated the Obama administration’s ability to navigate a crisis that presents an opportunity to remove a longtime U.S. adversary but carries the risk of bolstering insurgents sympathetic to al-Qaeda or militant Islam.

“But we’ve got to figure out who is over there first, and we don’t really know that,” said a U.S. official who expressed concern over persistent gaps and who, like others interviewed, spoke on the condition of anonymity because he was discussing intelligence matters. “It’s not like this is a new war. It’s been going on for 16 months.”

The lack of clarity has also fueled anxiety among U.S. allies in the region over who will control Syria if Assad falls. Even among Arab intelligence services eager to help rebels overthrow Assad, “the vetting process is still in the early stages,” said a Middle Eastern intelligence official, insisting on anonymity to discuss his country’s involvement in the Syrian crisis.

The foreign official cited concern that the opposition is at risk of becoming dominated by Islamists pushing for a Muslim Brotherhood government after Assad.

“We think this is a majority view, at least among those who are fighting in the streets,” the official said.

The CIA’s ability to operate inside Syria was hampered severely by the decision to close the U.S. Embassy in Damascus in February, officials said. Unlike in Libya, where rebels quickly seized control of the eastern half of the country, Syrian opposition groups have been unable to control territory that could serve as a foothold for CIA teams.

Despite the limitations, President Obama has given the agency authority to provide aid to anti-Assad forces through a collection of operations that require the president’s signature on a covert action “finding.”

The agency has supplied encryption-enabled communications gear to opposition groups, presumably enabling the United States to monitor their talks. A small team of six or so CIA officers based along the Turkey-Syria border has worked to vet opposition leaders and coordinate the flow of equipment and medical supplies, according to U.S. officials.

The agency has not been involved in supplying weapons, officials said, though the CIA has shared intelligence with countries, including Saudi Arabia and Qatar, that are providing arms to the rebels.

Other countries, including Jordan and the United Arab Emirates, have been willing to send money but not munitions, out of concern that the same firepower could eventually be turned against moderate Arab governments or Israel and possibly used against Syrian ethnic and religious minorities, U.S. and Middle Eastern officials said.

The concerns help explain why opposition groups remain outgunned despite backing from the United States and some of the region’s most powerful regimes.

Despite criticism from Republicans, the administration is reluctant to be drawn more deeply into another Middle East conflict. But in explaining their restraint, administration officials also cited the ongoing confusion about the composition of anti-Assad groups.

“The United States has a rather checkered history with arming opposition groups — we’re currently fighting one,” an administration official said, alluding to the decision in the 1980s to arm militias in Afghanistan that later morphed into al-Qaeda. “You really have to think hard about the second- or third-order effects of making that decision,” the official said, adding that in Syria “there could be a number of extremist elements.”

“The agency and others are trying to learn more about them,” the official said. “It’s still the case that without actual access to Syria, it’s hard to know exactly who they are.”

Seeking other ways to undermine Assad, the CIA and other spy agencies have expanded efforts to disrupt the flow of arms to the regime from Iran. Officials also cited the bombing in Damascus last week that killed four members of Assad’s inner circle as evidence that the opposition is increasingly capable even without lethal aid from the United States.

U.S. officials said intelligence on some aspects of the conflict in Syria has improved. Developments such as advances of rebel forces and defections within the nation’s military are being tracked remotely through satellite imagery and intercepted e-mails and calls.

U.S. intelligence analysts think that the bombing in Damascus was carried out by an insider with access to top Syrian national security officials. In contrast with the string of bombings earlier in the year, the latest attack has not been linked to al-Qaeda.

Although intelligence on the opposition is incomplete, “we know a lot more than we did,” said a senior administration official who helps oversee Syria policy. “We’re identifying the key leaders, and there are a lot of them. We are in touch with them, and we stay in touch.”

Others cast doubt on that assertion.

“The folks that have been identified have been identified through Turkey and Jordan,” another U.S. official said. “It's not because of who we know. It’s all through liaison.”

Secretary of State Hillary Rodham Clinton and other senior U.S. officials have met with some opposition leaders. But officials described the Free Syrian Army and other groups as a disorganized front, unlike the unified Transitional National Council in Libya.

In that country, the CIA had inserted teams within weeks of the outbreak of violence to contact opposition groups and, later, help secure chemical weapons sites. Syria, with backing from Iran, is considered a more formidable espionage adversary, and it is suspected of possessing more chemical weapons stockpiles than Libya had.

Congressional officials said they have pressed the CIA for details on its plan to protect chemical weapons sites in Syria but have been rebuffed.

“We keep asking questions,” said a congressional aide who was not authorized to publicly discuss sensitive intelligence matters. “We get nothing.”

The fear that allies of al-Qaeda might acquire such weapons in the chaos of a Damascus collapse is a major concern for U.S. counterterrorism officials. Al-Qaeda’s presence has expanded in Syria over the past six months, officials said. But it still represents a small fraction of the opposition to Assad, and there are indications that its fighters are no longer blending into the insurgency as seamlessly as they did at the start of the war.

The rebels have tried to keep their distance from al-Qaeda, leaving the group “disconnected from the rest of the opposition,” said a U.S. official familiar with recent CIA assessments.